The goals of this proposal are to continue trying to understand in molecular detail how retroviruses interact with their hosts to cause diseases, in particular, cancer. An intrinsic hope of this line of work is that this understanding will ultimately contribute to the prevention or treatment of viral induced diseases. The specific goals of the proposal are 1) To continue our detailed genetic and biochemical analysis of the enhancers of mouse retroviruses in order to understand how these elements interact with cellular proteins to encode biological properties of the viruses including the type of leukemia they induce. 2) To apply a similar analysis to the LTR of the simian AIDS virus SIV in order to learn whether this element also plays an important role in disease induction by the virus. 3) To continue trying to isolate genes that encode retrovirus receptors, particularly the MCF virus receptor, in order both to learn about virus-receptor interactions and to probe the role of MCF viruses in mouse retrovirus induced leukemias.